It is known to machine an electrode workpiece by an electrode tool, in EDM processes and apparatus, by feeding the electrode tool and the electrode workpiece relative to each other along a principal feed axis causing a penetration of the electrode tool into the workpiece to form a cavity while simultaneously orbiting the electrodes relative to each other along a pair of orthogonal axes disposed in a plane perpendicular to the principal feed axis, such as to overcut the electrode workpiece or, in other words, cut in the workpiece a cavity of a larger width than could be achieved by relative feed of the electrodes along the principal axis only, without relative orbiting motion of the electrodes. It is also known to vary the amplitude of the relative translation or orbiting motion as a function of the position of the electrode tool relative to the electrode workpiece along the principal feed axis, as disclosed in U.S. Pat. No. 3,539,754, to provide the workpiece with a cavity of a predetermined conicity relative to the lateral surface of the cavity. The amplitude of lateral translation of the electrode tool relative to the workpiece, or orbital motion, is progressively changed as a function of the amount of penetration of the electrode tool within the workpiece.
In the known EDM processes, the trace of trajectory of the electrode orbiting path is disposed along a conical surface of revolution inclined relative to the principal feed axis at the same angle as the surface machined in the workpiece and, when unfavorable machining conditions occur, a servo control causes the electrode tool to retract from within the cavity in the workpiece, the retraction of the electrode tool being effected along the conical surface. As a result, the machining gap width is increased only very slowly, which prevents eliminating abnormal machining conditions.
The present invention has for principal object to provide machining in a workpiece a cavity having a predetermined shape, while still permitting a rapid increase of the machining gap width when the electrode tool is retracted and sinking a relatively deep cavity in the workpiece, if so required.